This invention relates generally to the field of arc fault circuit interrupters, and more particularly to an arc fault circuit interrupter which detects upstream series arc faults during an interval shortly after power is applied to the arc fault circuit interrupter.
A percentage of fires each year are caused by electrical branch circuit wiring arcing faults involving currents below the trip level of a conventional circuit breaker or OCPD (over current protection device) as well as below the handling rate of the breaker. Basic overcurrent protection afforded by circuit breakers is designed to prevent I2R heating of the wiring in the electrical distribution system, caused by circuit overloading or line-to-line faults, and not necessarily arcing faults. A true short circuit is a rarity in an electrical system. In fact, it is more accurate to think of electrical faults as having some level of impedance, such as a high impedance arc fault (low current) or a low impedance fault (high current). Many electrical faults begin as high impedance breakdowns between the line and neutral conductors or to the ground wire or device components. AFCI (Arc Fault Circuit Interrupter) technology affords protection from conditions that may not necessarily be an immediate threat but could become hazardous if left unattended.
In order to start a fire, three elements must be present fuel, oxygen (air), and energy to ignite the fuel. Arcing is defined as a luminous discharge of electricity across an insulating medium. The electrical discharge of an arc can reach temperatures of several thousand degrees Celsius. Arcing produces sufficient energy to reach the ignition point of nearby combustible material(s) before a circuit breaker can respond. Arc detection is an enhancement to thermal magnetic overload detection typically used in circuit breakers or OCPD""s, which alone may not detect and respond to arc faults.
A number of devices for detecting arc faults and methods of detection have been used in the past. These include using E and B field arc sensors, detecting the amplitude of the rate of change of current signals when an arc fault occurs, using non-overlapping band pass filters to detect white noise characteristic of arcs, and utilizing the high frequency components (RF) of arcing waveforms to detect arcing faults. While some of these techniques are more or less effective than others, they require relatively sophisticated arc sensors and circuits. Heretofore, most arc detection circuits have been incorporated in circuit breakers.
xe2x80x9cA-typexe2x80x9d arc faults are those in which the arc occurs across a break in the line or neutral conductors or at a loose terminal in a branch circuit of a distribution network. The conductors are carrying current to a load derived from the line voltage. The arc could likewise occur as a break or at a loose terminal associated with an extension cord deriving power from line voltage, thereby completing the circuit to the load. Since the current through the A-type fault is limited by the impedance of the load itself, since the fault is in series with the load, an A-type fault is also known as a xe2x80x9cseries fault.xe2x80x9d
xe2x80x9cB-typexe2x80x9d arc faults are a second arcing condition that must be detected and interrupted by a combination outlet device. In a B-type fault, the arc occurs across two conductors in the branch circuit or extension cords plugged into it, at a site where the insulating media separating the two conductors has been compromised. The arc may occur across the line and neutral conductors or the line and ground conductors, or in the case of reverse polarity where the line voltage is reverse-polarized, between the neutral and ground conductors. The current through the B-type fault is not limited by the impedance of the load, but rather by the available current from the supply established by the impedance of the conductors and terminals between the source of line voltage and the position of the parallel fault, i.e., the conductive members carrying the fault current. Since B-type faults are effectively across the line, they are also known as xe2x80x9cparallel faults.xe2x80x9d
Many AFCI""s depend on an active power supply at the time of an arc fault. One example uses a timer to measure the width of the voltage half waves, since the voltage half waves shrink during an upstream series arc fault. However, when an arc fault occurs simultaneously with the powering up of the AFCI, the power supply is not active and cannot detect the arc fault. This prevents the AFCI from interrupting the load current feeding the series arc quickly enough to prevent ignition of nearby combustibles. In addition, when upstream series arcing is intermittent and has long gaps in-between reappearance of the line voltage on the load side of the series arc, an electronic detection method that depends on a charged power supply is disabled and fails to recognize the arc.
Briefly stated, line voltage pulses from an upstream series arc fault are passed through a high pass filter. The filtered output is rectified and used to pulse-charge a capacitor. When the capacitor charge reaches a predetermined threshold, a first switch is activated, which in turn activates an SCR to trigger a circuit interrupting mechanism which interrupts a current to a load, thereby interrupting the upstream series arc fault. A separate detection circuit which requires a DC power supply preferably disables the first switch after a predetermined time after appearance of normal line voltage absent upstream series arcing.
According to an embodiment of the invention, a method for detecting and interrupting an upstream series arc fault includes the steps of (a) high pass filtering line voltage pulses from an upstream series arc fault to produce a filtered output; (b) charging a capacitor with pulses from the filtered output; (c) activating a first switch when a charge on the capacitor reaches a predetermined threshold; (d) activating, in response to activation of the first switch, a second switch; and (e) interrupting, in response to activation of the second switch, current to a load, thereby interrupting the upstream series arc fault.
According to an embodiment of the invention, an apparatus for detecting and interrupting an upstream series arc fault includes filtering means for high pass filtering line voltage pulses from an upstream series arc fault to produce a filtered output; charging means for charging a capacitor with pulses from the filtered output; first activation means for activating a first switch when a charge on the capacitor reaches a predetermined threshold; second activation means for activating, in response to activation of the first switch, a second switch; and interrupting means for interrupting, in response to activation of the second switch, current to a load, thereby interrupting the upstream series arc fault.
According to an embodiment of the invention, an arc fault circuit interrupter receiving voltage from an AC power line having at least two line conductors includes line terminals for attaching the interrupter to the line conductors; load terminals for attaching the interrupter to a load, the load terminals being electrically connected to the line terminals; an interrupting mechanism for electrically disconnecting the load terminals from the line terminals; a detector for detecting a harmonic distortion in a voltage at the line terminals, the harmonic distortion being caused by an upstream series arc fault in one line conductor; and an integrator for establishing a duration of the detected harmonic distortion, wherein when the detected harmonic distortion is present for a pre-established duration, the interrupting mechanism trips to electrically disconnect the load terminals from the line terminal whereby a load current passing through the upstream series arc fault is stopped.
According to an embodiment of the invention, an arc fault circuit interrupter receiving voltage from an AC power line having at least two line conductors includes line terminals for attaching the interrupter to the line conductors; load terminals for attaching the interrupter to a load, the load terminals being electrically connected to the line terminals; an interrupting mechanism for electrically disconnecting the load terminals from the line terminals; a detector for detecting a harmonic distortion in a voltage at the line terminals, the harmonic distortion being caused by an upstream series arc fault in one line conductor; an integrator for establishing a duration of the detected harmonic distortion, and a hold circuit responsive to the voltage at the line terminals, wherein the voltage at the line terminals being above a threshold longer than a predetermined time enables the hold circuit, and wherein the enabled hold circuit prohibits the integrator from producing a signal to trip the interrupting mechanism, irrespective of a presence of the harmonic distortion.
According to an embodiment of the invention, an arc fault circuit interrupter receiving voltage from an AC power line having at least two line conductors includes line terminals for attaching the interrupter to the line conductors; load terminals for attaching the interrupter to a load, the load terminals being electrically connected to the line terminals; an interrupting mechanism for electrically disconnecting the load terminals from the line terminals; a detector for detecting a harmonic distortion in a voltage at the line terminals, the harmonic distortion being caused by an upstream series arc fault in one line conductor; an integrator for establishing a duration of the detected harmonic distortion, and a hold circuit responsive to the voltage at the line terminals, wherein the voltage at the line terminals being above a threshold longer than a predetermined time enables the hold circuit, and wherein the enabled hold circuit prohibits the detector from producing a signal to the integrator to trip the interrupting mechanism, irrespective of a presence of the harmonic distortion.
According to an embodiment of the invention, an arc fault circuit interrupter receiving voltage from an AC power line having at least two line conductors includes line terminals for attaching the interrupter to the line conductors; load terminals for attaching the interrupter to a load, the load terminals being electrically connected to the line terminals; an interrupting mechanism for electrically disconnecting the load terminals from the line terminals; an integrator for establishing a duration of the detected harmonic distortion, first and second detectors for detecting a harmonic distortion in a voltage at the line terminals; first and second timers connected to first and second detectors, respectively, to determine if detected harmonic distortions exceed first and second pre-established intervals; first and second hold circuits responsive to a derived voltage derived from the voltage at the line terminals, wherein the first detector and the first timer are enabled by the first hold circuit if the derived voltage is less than a first predetermined threshold, and wherein the second detector and the second timer are enabled by the second hold circuit if the derived voltage is greater than a second pre-determined threshold; and wherein when detected harmonic distortion exceeds the first pre-established interval, the first timer produces a signal to trip the interrupting mechanism, and when detected harmonic distortion exceeds the second pre-established interval, the second timer produces a signal to trip the interrupting mechanism.